1. Field of the Invention
The present invention relates to a control system for distributing braking forces applied to front and rear vehicle wheels so as to prevent the vehicle from moving along an unacceptable turning locus, when the vehicle is turned to the right or the left.
2. Description of the Background Art
As is generally known, hydraulic type brake systems use the pressure of a working fluid (brake fluid) to force the brake shoes against the brake drums or disk. In such traditional hydraulic brake systems brake fluid pressure is forced through a master cylinder to each wheel-cylinder in proportion to a depressing force exerted on a brake pedal and as a result each vehicle wheel is braked. During braking, more of the car weight is transferred to the front wheels and thus the car weight becomes less at the rear wheels. If normal braking were continued with the front-wheel brake fluid pressure equal to the rear-wheel brake fluid pressure, the brakes could first lock the rear wheels so that the rear tires skid. As a result, the rear wheels could throw the entire car into a rear-end skid. This could result in oversteer and/or spinning on wet or icy roads. As is well known, it is advantageous to provide a braking action according to which front wheel lock gets priority over rear wheel lock so as to prevent oversteer tendencies on turns. As indicated by a broken line of FIG. 6, it is desired that both front and rear wheel brake fluid pressures are varied in accordance with an ideal brake fluid pressure distribution characteristic curve wherein both front and rear wheels are locked simultaneously so as to provide an optimal braking efficiency.
In view of the above, as shown by a solid line corresponding to a brake fluid pressure characteristic curve a-b-c in FIG. 6, a conventional hydraulic brake system employs a brake fluid control valve through which the rear-wheel brake fluid pressure P.sub.R is set to a lower level over a wide range across a preset point b (a preset brake fluid pressure P.sub.S1) of the ideal brake fluid pressure distribution characteristic curve indicated by the above noted broken line. The point b corresponds to a particular point across which a brake fluid pressure distribution between front and rear wheels is essentially varied. As appreciated from the brake fluid pressure characteristic curve a-b-c in FIG. 6, the conventional fluid pressure control valve controls the brake fluid pressure distribution between front and rear wheels such that the rear-wheel brake fluid pressure P.sub.R is set essentially at the same value as the front-wheel brake fluid pressure P.sub.F in a relatively low fluid pressure range from a to b and held substantially constant at a lower level than the front-wheel brake fluid pressure in a relatively high fluid pressure range from b to c. As seen in FIG. 6, the brake fluid pressure distribution between the front and rear brake fluid pressures P.sub.F and P.sub.R is varied across the split point P.sub.S1 with a predetermined ratio of line bc to line afb. The point b is in general referred to as a "split point". A fluid pressure P.sub.S1 corresponding to the split point b is referred to as a "critical brake fluid pressure". As shown in FIG. 6, in a relatively high fluid pressure range exceeding the split point b, the rear-wheel brake fluid pressure P.sub.R is set at a lower pressure than the front-wheel brake fluid pressure P.sub.F, so as to avoid vehicle spin during braking on hard turns. In other words, the brake fluid pressures in the front and rear wheel-cylinders are set such that the front wheel lock gets priority over the rear wheel lock to avoid oversteer tendencies of the vehicle during braking on turns.
However, in such a conventional hydraulic brake system achieving braking control according to the brake fluid pressure characteristic curve a-b-c, if the brakes are moderately applied to vehicle wheels when the vehicle turns with a relatively large lateral acceleration exerted on the vehicle body, the car weight is shifted to the front wheels. Since neither front nor rear wheels reach a locking condition due to such moderate braking action, cornering force created at the front wheels is increased, whereas cornering force created at the rear wheels is decreased, thereby resulting in oversteer during moderate braking operations on hard turns. On the other hand, in the conventional hydraulic brake system having the brake fluid pressure characteristics a-b-c, if the brakes are quickly applied to vehicle wheels during hard turn, the vehicle may experience front-end skid because the front wheels are first locked due to the front-wheel brake fluid pressure P.sub.F being set to a higher level than the rear-wheel brake fluid pressure P.sub.R. There is then a tendency for understeer to occur during hard braking on a severe turn.